fix(graph): make decode capture batch sizes configurable

[gongchensu]

Summary

• Add a configurable graph_capture_batch_sizes option for paged decode graph capture.
• Propagate the option through the Python API, server configuration, pybind layer, and C++ engine configuration.
• Validate, sort, and deduplicate explicitly configured batch sizes.
• Preserve the existing default capture policy when the option is not specified.
• Fall back to eager execution when the runtime batch size has no captured graph.

Motivation

The default paged decode graph policy captures every batch size from 1 to the configured maximum batch size.

For the Iluvatar BI-V150 test case with --batch-size=16, this produced 16 batch-size variants. With 33 decode graph segments per variant, the engine retained 528 graph instances and ran out of device memory during the benchmark.

This change allows deployments to capture only the batch sizes they expect to use. Capturing only batch size 16 reduced this test from 528 to 33 graph instances and allowed all 12 workloads in test case 3 to complete.

The default behavior remains unchanged when graph_capture_batch_sizes is not specified. Runtime batch sizes without a captured graph use the existing eager execution path.

Related issue: N/A — identified during Iluvatar backend validation.

Type of Change

• feat — new feature / new model
• fix — bug fix
• perf — performance improvement (no behavioral change)
• refactor — code restructuring without behavior change
• test — adding or fixing tests only
• docs — documentation only
• build / ci — build system or CI configuration
• chore — tooling, formatting, or other non-code changes
• Breaking change

Test Results of Involved Models on Supported Platforms (Please attach screenshots)

Benchmark / Performance Impact

Notes for Reviewers

CI / ChatOps

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